Field
This invention relates to optical devices and more particularly relates to multilevel leaky-mode resonant reflectors, polarizers, and beamsplitters.
Description of the Related Art
Multilayer thin films are widely applied to implement filters, polarizers, and reflectors for incorporation in various common optical systems. These devices are typically stacks of homogeneous layers deposited with precise thicknesses and indices of refraction and absorption. In many cases, a large number of layers, perhaps 10-100, may be needed to create the spectral, polarization, and angular attributes required for a particular application. These optical devices operate on the basis of multiple reflections between the interfaces incorporated in a layer stack. In particular, periodic quarter-wave layer systems provide classical high reflectors for bulk laser cavities as well as integrated distributed Bragg reflectors for vertical cavity lasers. Bragg reflectors yield efficient reflection across wide spectral bands [H. A. Macleod, Thin-Film Optical Filters, (McGraw-Hill, New York, 1989)]. Additionally, subwavelength periodic layers exhibit strong resonance effects that originate in quasi-guided, or leaky, waveguide modes. These compact elements yield versatile photonic spectra [E. Popov, L. 5 Mashev, and D. Maystre, “Theoretical study of anomalies of coated dielectric gratings,” Opt. Acta 33, 607-619 (1986).; G. A. Golubenko, A. S. Svakhin, V. A. Sychugov, and A. V. Tishchenko, “Total reflection of light from a corrugated surface of a dielectric waveguide,” Sov. J. Quantum Electron. 15, 886-887 (1985).; I. A. Avrutsky and V. A. Sychugov, “Reflection of a beam of finite size from a corrugated waveguide,” J. Mod. Opt. 36, 1527-1539 (1989).; S. S. Wang and R. Magnusson, “Theory and applications of guided-mode resonance filters,” Appl. Opt. 32, 2606-2613 (1993).].
The functionality and applicability of thin films in optics and photonics technology can be complemented and enhanced by imbuing them with appropriate periodic modulation to achieve leaky-mode resonance. For example, it has been shown that a single periodic layer with one-dimensional periodicity enables narrow-line filters, polarizers, reflectors, and polarization independent elements [Y. Ding and R. Magnusson, “Resonant leaky-mode spectral-band engineering and device applications,” Opt. Express 12, 5661-5674 (2004)]. Additionally, tunable filters and display pixels are feasible as discussed in R. Magnusson and Y. Ding, “MEMS tunable resonant leaky mode filters,” IEEE Photonics Technol. Lett. 18, 1479-1481 (2006); R. Magnusson and M. Shokooh-Saremi, “Widely tunable guided-mode resonance nanoelectromechanical RGB pixels,” Opt. Express 15, 10903-10910 (2007).
Efficient reflection of light across wide spectral bands is essential in a plethora of common photonic systems. Classic mirrors are made with evaporated metal films and dielectric multilayer stacks. These ordinary devices have been widely studied for a long time and are well understood. A new method to achieve effective wideband reflection response has recently emerged. This approach is based on leaky-mode resonance effects in single-layer, high refractive index contrast, one-dimensional (1D) and two-dimensional (2D) waveguide gratings. Briefly reviewing the relevant work and literature, the pursuit of resonant wideband response can be traced to Gale et al. [M. T. Gale, K. Knop, and R. Morf, “Zero-order diffractive microstructures for security applications,” Proc. SPIE 1210, 83-89 (1990).] and to Brundrett et al. [D. L. Brundrett, E. N. Glytsis, and T. K. Gaylord, “Normal-incidence guided-mode resonant grating filters: design and experimental demonstrations,” Opt. Lett. 23, 700-702 (1998).] who achieved experimental full-width half-maximum (FWHM) linewidths near 100 nm albeit not for flat spectra. Applying cascaded resonance structures, Jacob et al. designed narrow-band flattop filters that exhibited lowered sidebands and steepened stopbands [D. K. Jacob, S. C. Dunn, and M. G. Moharam, “Normally incident resonant grating reflection filters for efficient narrow-band spectral filtering of finite beams,” J. Opt. Soc. Am. A 18, 2109-2120 (2001).]. Alternatively, by coupling several diffraction orders into corresponding leaky modes in a two-waveguide system, Liu et al. found a widened spectral response and steep filter sidewalls generated by merged resonance peaks [Z. S. Liu and R. Magnusson, “Concept of multiorder multimode resonant optical filters,” IEEE Photonics Technol. Lett. 14, 1091-1093 (2002).]. Suh et al. designed a flattop reflection filter using a 2D-patterned photonic crystal slab [W. Suh and S. Fan, “All-pass transmission or flattop reflection filters using a single photonic crystal slab,” Appl. Phys. Lett. 84, 4905-4907 (2004).]. Emphasizing new modalities introduced by asymmetric profiles, Ding et al. presented single-layer elements exhibiting both narrow and wide flat-band spectra [Y. Ding and R. Magnusson, “Use of nondegenerate resonant leaky modes to fashion diverse optical spectra,” Opt. Express 12, 1885-1891 (2004).]. Using a subwavelength grating with a low-index sublayer on a silicon substrate, Mateus et al. designed flattop reflectors with linewidths of several hundred nanometers operating in TM polarization [C. F. R. Mateus, M. C. Y. Huang, Y. Deng, A. R. Neureuther, and C. J. Chang-Hasnain, “Ultrabroadband mirror using low-index cladding subwavelength grating,” IEEE Photonics Technol. Lett. 16, 518-520 (2004).]. Subsequently, they fabricated a reflector with reflectance exceeding 98.5% over a 500 nm range and compared the response with numerical simulations [C. F. R. Mateus, M. C. Y. Huang, L. Chen, C. J. Chang-Hasnain, and Y. Suzuki, “Broad-band mirror (1.12-1.62 μm) using a subwavelength grating,” IEEE Photonics Technol. Lett. 16, 1676-1678 (2004).]. Ding et al. showed single-layer elements with about 600 nm flattop reflectance in both TE and TM polarization [Y. Ding and R. Magnusson, “Resonant leaky-mode spectral-band engineering and device applications,” Opt. Express 12, 5661-5674 (2004).]. Most recently, Magnusson et al. provided detailed physical basis for such broadband leaky-mode reflectors by treating the simplest possible case, which was a single-layer, one-dimensionally patterned waveguide grating [R. Magnusson and M. Shokooh-Saremi, “Physical basis for wideband resonant reflectors,” Opt. Express 16, 3456-3462 (2008).].